netdev - Re: [PATCHv3 net-next 1/2] net: move the hsize check to the else block in skb

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Date:   Fri, 15 Jan 2021 17:39:02 -0800
From:   Alexander Duyck <alexander.duyck@...il.com>
To:     Xin Long <lucien.xin@...il.com>
Cc:     network dev <netdev@...r.kernel.org>,
        "linux-sctp @ vger . kernel . org" <linux-sctp@...r.kernel.org>,
        Marcelo Ricardo Leitner <marcelo.leitner@...il.com>,
        Neil Horman <nhorman@...driver.com>,
        David Miller <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>
Subject: Re: [PATCHv3 net-next 1/2] net: move the hsize check to the else
 block in skb_segment

On Fri, Jan 15, 2021 at 1:36 AM Xin Long <lucien.xin@...il.com> wrote:
>
> After commit 89319d3801d1 ("net: Add frag_list support to skb_segment"),
> it goes to process frag_list when !hsize in skb_segment(). However, when
> using skb frag_list, sg normally should not be set. In this case, hsize
> will be set with len right before !hsize check, then it won't go to
> frag_list processing code.
>
> So the right thing to do is move the hsize check to the else block, so
> that it won't affect the !hsize check for frag_list processing.
>
> v1->v2:
>   - change to do "hsize <= 0" check instead of "!hsize", and also move
>     "hsize < 0" into else block, to save some cycles, as Alex suggested.
>
> Signed-off-by: Xin Long <lucien.xin@...il.com>

Looks good to me.

Reviewed-by: Alexander Duyck <alexanderduyck@...com>

> ---
>  net/core/skbuff.c | 11 ++++++-----
>  1 file changed, 6 insertions(+), 5 deletions(-)
>
> diff --git a/net/core/skbuff.c b/net/core/skbuff.c
> index 6039069..e835193 100644
> --- a/net/core/skbuff.c
> +++ b/net/core/skbuff.c
> @@ -3894,12 +3894,8 @@ struct sk_buff *skb_segment(struct sk_buff *head_skb,
>                 }
>
>                 hsize = skb_headlen(head_skb) - offset;
> -               if (hsize < 0)
> -                       hsize = 0;
> -               if (hsize > len || !sg)
> -                       hsize = len;
>
> -               if (!hsize && i >= nfrags && skb_headlen(list_skb) &&
> +               if (hsize <= 0 && i >= nfrags && skb_headlen(list_skb) &&
>                     (skb_headlen(list_skb) == len || sg)) {
>                         BUG_ON(skb_headlen(list_skb) > len);
>
> @@ -3942,6 +3938,11 @@ struct sk_buff *skb_segment(struct sk_buff *head_skb,
>                         skb_release_head_state(nskb);
>                         __skb_push(nskb, doffset);
>                 } else {
> +                       if (hsize > len || !sg)
> +                               hsize = len;
> +                       else if (hsize < 0)
> +                               hsize = 0;
> +
>                         nskb = __alloc_skb(hsize + doffset + headroom,
>                                            GFP_ATOMIC, skb_alloc_rx_flag(head_skb),
>                                            NUMA_NO_NODE);
> --
> 2.1.0
>